1. Technical Field
The present invention relates to a rubber composition and a pneumatic tire using the rubber composition
2. Related Art
A rubber composition forming a tread part, side wall part and rim strip of a pneumatic tire contains a wax in order to suppress deterioration by ozone in the air and ultraviolet rays. Wax has a deterioration suppressing effect, and on the other hand, whitens a rubber surface, becoming a factor of poor appearance of a tire.
To suppress whitening of a side wall part by a wax, JP-A-2014-210830 proposes to add a fatty acid metal salt having the carbon number of from 16 to 20 smaller than the carbon number of a hydrocarbon most contained in a wax, together with a wax. JP-A-2008-095028 proposes to add an aromatic oil having an average molecular weight of from 320 to 420 together with a hydrocarbon wax having a specific carbon number distribution in order to improve ozone resistance and appearance in good balance. However, it has been not known that black and luster can be given while preventing whitening, by adding a processing aid having a specific carbon number distribution together with cashew oil.
JP-A-2010-053282 discloses to add cashew oil to a rubber composition for a tire in order to improve rolling resistance performance, abrasion resistance, crack resistance and the like. However, this patent document does not disclose to use the cashew oil together with a processing aid having a specific carbon number distribution.
JP-A-2005-206673 discloses to add a fatty acid metal salt, a fatty acid amide and/or fatty acid ester as processing aids together with a silane coupling agent to a rubber composition for a tire tread in order to balance processability and tire performance in high dimension in the case of adding an inorganic filler having a hydroxyl group, such as silica. However, this patent document does not disclose to further use cashew oil together, and does not suggest in any fashion that the processing aid having a specific carbon number distribution exhibits excellent effect by using together with cashew oil.